The present invention relates to heart pacemakers, and in particular to a heart pacemaker having the capability of controlling the output pulse rate of the pacemaker in accordance with changing respiration characteristics of the pacemaker user.
During respiration of a patient, the electrical impedance between two electrodes on the chest of the patient changes. By monitoring this impedance, it is thus possible to calculate the respiration rate and deepness of breathing of the patient. The ventilated breath volume per minute can in turn be calculated therefrom.
It is an object of the present invention to provide a heart pacemaker having the capability of forming electrical signals which are dependent upon the respiration of the pacemaker user, and employing the respiration signals for physiological control of the pacemaker output.
The above object is achieved in accordance with the principles of the present invention by a pacemaker having circuitry for forming electrical signals corresponding to the stimulation current and to the stimulation voltage. The pacemaker includes a divider for these signals which forms an impedance signal corresponding to the electrode impedance. The impedance signal is supplied to a control stage for the pacemaker pulse generator after a signal component dependent upon the respiration has been filtered therefrom. The portions of the impedance signal dependent upon the respiration of the patient can then be employed for theraputically controlling the pulse generator of the pacemaker. In contrast to conventional respiration methods, the respiration signal in the pacemaker described herein is acquired from the stimulation pulses by the standard stimulation electrode, without the use of additional measuring electrodes and the accompanying additional measuring energy which is necessary in known devices.
At rest, normal respiration rates are between 10 and 20 respiratory cycles per minute. Because the normal heart pacemaker stimulation rate is about 70 stimulation pulses per minute, roughly 5 measuring events for the electrode impedance can be executed per respiration cycle. This is entirely adequate for acquiring the respiration rate and the volume of breath per minute.
In a heart pacemaker wherein the stimulation rate is variable, the relationship of the heart stimulation rate and respiration can be maintained given intensification of respiration (increase in the respiratory rate). It is also possible to employ a varying sampling rate for measuring the respiration parameters.